Most of the industry standard ion detectors in mass spectrometers are equipped with high voltage conversion dynodes to enhance ion detection, especially for ions having high molecular masses. Ions exiting from a mass analyzer, such as a quadrupole mass filter, are projected to a high voltage conversion dynode so that their collisions with the dynode cause secondary charged particles to be radiated from the dynode surface. These secondary charged particles are repelled by the dynode so as to direct and focus them into the input port of an electron multiplier (e.g., either continuous channel or discrete dynode construction) in order to generate an electrical pulse for further signal processing. Additional ion optic lenses may be installed to increase ion collection from the mass analyzer.
In mass spectrometers, the conversion dynode is positioned such that the axis of symmetry of the ion impact region, on the dynode surface, intersects with the axis of the mass analyzer ion exit aperture. If long-lived, excited or metastable neutrals, which are created during an ionization process, are present among the ions exiting a mass filter, a noise signal is generated under the influence of the conversion dynode high voltage. Metastable neutrals, such as excited helium atoms, for instance, may ionize molecular background gas or may convert to ions under the influence of the conversion dynode high voltage. These ions then strike the dynode surface. This action generates unwanted electrical signal and thereby reduces the signal-to-noise ratio, and thus sensitivity, of the ion detector.
A small aperture may be installed at the ion exit of a mass analyzer to minimize the neutral noise. However, this method will also restrict the ions exiting from the mass analyzer and reduce the ion collection. Improvement of sensitivity, using this method, may not be significant.
The ion detector is one of the crucial components of mass spectrometers of the quadrupole, ion trap, or magnetic sector type, for instance. Electron multipliers, of either the continuous channel or discrete dynode type, have been utilized in ion detectors. It is very desirable to have high signal-to-noise ratios, or high sensitivity, for an ion detector. In industrial standard configurations, high voltage conversion dynodes are typically used to enhance ion collection and ion detection. This is especially true in applications where high molecular masses are able to generate more secondary charged particles due to higher energy collisions with the surface of the dynode. In an effort to increase detector sensitivity, the electron multiplier can be biased as high as the conversion dynode but this has proven to be impractical.
In general, a mass spectrometer, such as a quadrupole type as shown in FIG. 1, includes an ion source 1.1, a mass analyzer 1.2, and an ion detector 1.10. The conversion dynode 1.7 is positioned such that the axis passing through the center of the ion collision point 1.9 and perpendicular to the dynode collision surface 1.8 intersects with the longitudinal axis 1.6 of the ion beam exiting from the mass analyzer 1.2. That is, the axis collinear to the conversion dynode region 1.9 and the input port of the electron multiplier 1.11 intersects the longitudinal axis 1.6 of the ion beam exiting from the mass analyzer. An output plate 1.3 having an aperture may be used to maximize ion throughput. The ions from mass analyzer 1.2, with or without additional ion optics components 1.4, are projected to the dynode surface 1.8 and generate secondary charged particles which are repelled and focused into an input side of an electron multiplier 1.11. An electrical signal is generated after an electron multiplication process.
FIG. 2 shows a conventional ion trap type mass spectrometer including an ion source 2.1, a mass analyzer 2.2, and an ion detector 2.8. The conversion dynode 2.4 is positioned such that the axis that passes through the center of the ion collision position 2.6 perpendicular to the dynode collision surface 2.5 intersects with the longitudinal axis 2.3 of the ion beam exiting the mass analyzer 2.2. That is, the axis 2.7 collinear to the conversion dynode region 2.6 and the input port of the electron multiplier 2.9 intersects the longitudinal axis 2.3 of the ion beam exiting from the mass analyzer. The ions from mass analyzer 2.2, with or without additional ion optics components, are projected to the dynode surface 2.5 and generate secondary charged particles which are repelled and focused into an input side of an electron multiplier 2.9. An electrical signal is generated after an electron multiplication process.
Excited neutrals, such as metastable helium, can be created in an ionization process. If any such neutrals are present at the ion exit of a mass analyzer, neutral noise will be generated. Energetic metastable neutrals may ionize molecular background gas, and it is believed they may become ions under the influence of high voltage or a high electrical field. These ions are vigorously drawn to the surface of the conversion dynode and produce unwanted secondary charged particles. This effect contributes to neutral noise in a mass spectrum. The ion detector of FIG. 1 includes a small aperture ion optics lens assembly 1.4 to limit the neutral stream entering the conversion dynode region. This same detector also has a back-end aperture hole 1.5 in the conversion dynode enclosure to provide an escape pathway for neutrals such as metastable helium. These two features provide a method to reduce neutral noise, but at the cost of reduced ion collection efficiency resulting from lens aperture constrictions.
There is a need for an ion detector that suppresses neutral noise and improves ion detection sensitivity.